Category: SQL

As long as your data contains only the date portion, your queries will work as expected. However, if a time portion is involved, it gets complicated. Before talking about the complications of querying for dates, we will look at the most important built-in functions for working with dates. MySQL Date Functions The following table lists the most important built-in date functions in MySQL: Function Description NOW() Returns the current date and time CURDATE() Returns the current date CURTIME() Returns…

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A view is a virtual table. This chapter shows how to create, update, and delete a view. SQL CREATE VIEW Statement In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the…

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Auto-increment allows a unique number to be generated when a new record is inserted into a table. AUTO INCREMENT a Field Very often we would like the value of the primary key field to be created automatically every time a new record is inserted. We would like to create an auto-increment field in a table. Syntax for MySQL The following SQL statement defines the “ID” column to be an auto-increment primary key field in the “Persons” table: CREATE TABLE…

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The ALTER TABLE statement is used to add, delete, or modify columns in an existing table. SQL ALTER TABLE Syntax To add a column in a table, use the following syntax: ALTER TABLE table_name ADD column_name datatype To delete a column in a table, use the following syntax (notice that some database systems don’t allow deleting a column): ALTER TABLE table_name DROP COLUMN column_name To change the data type of a column in a table, use the following syntax:…

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Indexes, tables, and databases can easily be deleted/removed with the DROP statement. The DROP INDEX Statement The DROP INDEX statement is used to delete an index in a table. DROP INDEX Syntax for MS Access: DROP INDEX index_name ON table_name DROP INDEX Syntax for MS SQL Server: DROP INDEX table_name.index_name DROP INDEX Syntax for DB2/Oracle: DROP INDEX index_name DROP INDEX Syntax for MySQL: ALTER TABLE table_name DROP INDEX index_name   The DROP TABLE Statement The DROP TABLE statement is…

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The DEFAULT constraint is used to insert a default value into a column. The default value will be added to all new records, if no other value is specified. SQL DEFAULT Constraint on CREATE TABLE The following SQL creates a DEFAULT constraint on the “City” column when the “Persons” table is created: My SQL / SQL Server / Oracle / MS Access: CREATE TABLE Persons ( P_Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City…

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SQL FOREIGN KEY Constraint A FOREIGN KEY in one table points to a PRIMARY KEY in another table. Let’s illustrate the foreign key with an example. Look at the following two tables: The “Persons” table: P_Id LastName FirstName Address City 1 Hansen Ola Timoteivn 10 Sandnes 2 Svendson Tove Borgvn 23 Sandnes 3 Pettersen Kari Storgt 20 Stavanger The “Orders” table: O_Id OrderNo P_Id 1 77895 3 2 44678 3 3 22456 2 4 24562 1 Note that the…

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SQL PRIMARY KEY Constraint The PRIMARY KEY constraint uniquely identifies each record in a database table. Primary keys must contain unique values. A primary key column cannot contain NULL values. Each table should have a primary key, and each table can have only ONE primary key. SQL PRIMARY KEY Constraint on CREATE TABLE The following SQL creates a PRIMARY KEY on the “P_Id” column when the “Persons” table is created: MySQL: CREATE TABLE Persons ( P_Id int NOT NULL,…

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